A basic requirement of flow regulation in a Kaplan turbine is to be able to close the inlet guide vanes LE by means of turning them around the axis D.sub.L.
Until now it was believed that this requirement could only be ensured by means of guide vanes of cylindrical shape, because only then would one have touching lines of overlapping guide vanes in the form of cylinder-generating straight lines E1 and E2 which are a part of the profile of the cylinder.
On the other hand a design of vanes and blades according to the potential flow theory is most promising for best efficiency because rotc=0 cannot be achieved by means of cylindrical guide vanes.
The cylindrical guide vane requires a constant distribution of the meridian velocity component along axis D.sub.L and projected perpendicular to the axis D.sub.L, which constant distribution is not possible due to the strongly curved 90.degree. deflection of the stream shroud contour, which deflects the stream lines.
For the accelerating flow in turbine vane cascades the application of a stream line picture based on the potential flow theory is a good approximation of the real flow if one considers the strongly varying meridian velocity component not only along the leading edge and trailing edge of the runner vanes but also along leading and trailing edges of the inlet guide vanes.
A design based upon a potential vortex motion (rotc=0) promises best efficiency and causes a constant value of angular momentum r*C.sub.u =const. in the space between runner vanes and guide vanes.
On the other hand, the guide vanes should also be able to close the flow. Published designs with axis D.sub.L parallel to D.sub.T do not have twisted guide vanes.
Since the invention of Kaplan turbines, those with a vertical axis and radial inflow have been made with cylindrical guide vanes.
Because Kaplan turbines with cylindrical guide vanes might reach high overall efficiencies (up to 94%) the cylindrical guide vanes in general were accepted